1. Field of the Invention
The present invention relates to delay circuits, and more particularly to a delay circuit including a resistor and a capacitor.
2. Description of Related Art
FIG. 1 shows an example of a conventional delay circuit, which has an input terminal 10 and an output terminal 11. The input terminal 10 is connected to a gate G of an N-channel MOS transistor Q11 via a delay circuit. The delay circuit is composed of a resistor R11 and a capacitor C11. A drain D of the N-channel MOS transistor Q11 is connected to a node 13 via a resistor R12. The node 13 is at high level. The drain D of the N-channel MOS transistor Q11 is connected to the output terminal 11. A source S of the N-channel MOS transistor Q11 is connected to ground.
Referring to FIG. 2, at t=T1a, a signal at the input terminal 10 changes from a low level to a high level. The capacitor C11, which is connected to the input terminal 10, charges. The voltage at the gate G of the N-channel MOS transistor Q11 gradually rises with the charging of the capacitor C11. A signal at the output terminal 11 is high. At t=T2a, the gate voltage of the N-channel MOS transistor Q11 is 2 (V), which turns on the N-channel MOS transistor Q11. At this time, the signal at the output terminal 11 become low. The voltage level change of the signal at the output terminal 11 is delayed by (T2a−T1a) compared to the voltage level change of the signal at input terminal 10. At t=T3a, the signal at input terminal 10 changes from high to low. At this time, the capacitor C11, discharges. The voltage at the gate G of the N-channel MOS transistor Q11 gradually drops with the discharging of the capacitor C11. At t=Ta4, the gate voltage of the N-channel MOS transistor Q11 will be below 2 (V), which will turn off the N-channel MOS transistor Q11. The voltage level change of the signal at the output terminal 11 is delayed by (T4a−T3a) compared to the voltage level change of the signal at the input terminal 10. However, in some circuits, users need to discharge the capacitor immediately.
What is needed, therefore, is a delay circuit which can discharge the capacitor immediately.